This invention relates to a concentric holdown connection for anchoring a first building structural member or held member to a second building structural member such as a foundation or holding member.
The concentric holdown connection includes a holdown connector and an anchor member that is connected to the holding member, The holdown connector is attached to the held member with fasteners. The anchor member is threadably connected to a combination member seated on the holdown connector.
Holdown connectors have been used for many years in building structures to strengthen the joints of wood frame members to better withstand such cataclysmic forces such as earthquakes, hurricanes, tornadoes, and floods. One of the primary uses of holdowns is in connecting the frame of a building to the concrete foundation.
Early holdowns and even most holdowns in use today are formed so that there is a short but deleterious lateral distance between the load force applied by the held member and the resistance force applied by the holding member causing an eccentric loading. This eccentric loading applies a load multiplier effect which is directly related to the distance between the applied load and the resistance load. Such holdown are hereafter referred to as eccentric holdowns.
Within the past few years a form of holdown known as a concentric holdown has been developed. In a concentric holdown, the applied load and resistant load are on or as near as possible to the same axis. The holdown of the present invention is another form of concentric holdown.
A recent example of a concentric holdown is found in U.S. Pat. No. 6,513,290 granted to William F. Leek on Feb. 4, 2003.
The holdown of the present invention improves upon the prior art holdowns by providing a combination nut, washer, clamp, and tension force flow member, hereafter sometimes referred to as a combination member. The combination member serves as a nut for threadably engaging the end of the threaded anchor member as well as serving as a washer for transferring the load forces from the holdown connector seat to the anchor member.
But, the combination member of the present invention is more than simply a washer. Additionally, the combination members serves as a tension force flow director.
U.S. Pat. No. 4,603,531 granted to Nash Aug. 5, 1986 shows a threaded nut 26 which also serves as a washer, but the threaded nut 26 serves only as a clamp by means of a bolt 25 to a specially formed foundation plate 28 which in turn is connected to the foundation by anchor bolts 30. (See FIGS. 4, 6, and 9 of U.S. Pat. No. 4,603,531)
Referring to FIG. 3 of Nash, the sides of elongated nut 26 have no part in directing the flow of tension forces from the seat penetrated by opening 24 of mounting bracket 20 around the right angle bend to the upright members of mounting bracket 20 which hold wood vertical member 10.
Millions of sheet metal eccentric holdowns have been successfully used in building structures in the United States since they were first introduced in the late 1970's and became commonly used during the 1980's.
Both eccentric and concentric holdowns are attached to large column type members such as 4×4's or even tripled 2×4's.
The initial and primary purpose of both eccentric and concentric holdowns is to hold the weight of the building frame and the contents of the building and transfer these compression forces to the concrete foundation. The best way to transfer these large compression forces is to set the ends of the wood members on a flat metal plate or flat connector seat.
When an earthquake, hurricane or tornado strikes, the downward forces suddenly are reversed and an equally important purpose of both eccentric and concentric holdown members is to resist upward forces and to prevent the frame of a building from lifting off its foundation. It is here that the concentric holdowns are generally superior to the eccentric holdowns because the momentum forces are minimized.
But even eccentric holdowns must be carefully designed. The upward force in a prior art concentric holdown is initially resisted by the upright portion or straps of the concentric holdown connector which pull upwards on the outside edges of the flat holdown seat. These upward forces, if unrestrained, would quickly bend a prior art holdown connector seat into the shape of the letter “U” and crush the cross grain wood mud sill members when the weight of the building crashes back down on the mud sill resulting in failure of the connection and possibly leading to the destruction of the building.
Some work has been done to improve the flow of forces through holdown connector members from the held member to the foundation member. Holdowns such as U.S. Pat. No. 5,979,130 granted Nov. 9, 1999 to Gregg, Leek and Commins have been constructed with pre-bent concave seats and provided with U-shaped washers.
The problem with the U-shaped seat connector taught by U.S. Pat. No. 5,979,130, however, is the fact that they are problematical for resisting compression loads. The compression loads are exerted on a very small surface area which results in crushing the mud sill instead of being distributed over a large area.
To spread the compression bearing load in the present invention, the connector seat is not curved, but rather is flat to spread the bearing compression loads.
The flow of forces which move from the flat seat to the vertical members around nearly a right angle where the straps meet the connector seat requires a force flow director. Channeling forces through a right angle bend is clearly the worst possible condition. Failure of prior art holdown connectors is often at the right angle bend between the seat and the upright members or at the anchor bolt opening in the seat.
Applicant teaches a structure which directs the huge forces imposed by earthquakes and hurricanes to flow from the vertical members of the connector straps to the flat seat member in a smooth curvilinear stream.
Applicant cannot use a connector with a U-shape bend and an n-shaped washer as taught in Gregg et. al. in U.S. Pat. No. 5,979,130, because the combination member of the present invention must be rotated relative to the holdown connector to tighten down the combination member on the threaded anchor member in a clamping force against the seat of the holdown member. Gregg et. al.'s n-shaped washer cannot be rotated.
The n-shaped washers of Gregg et. Al in U.S. Pat. No. 5,979,130 are unusable in Applicant's concentric holdown because they are operational in only one specific orientation.
Holdowns which can withstand huge forces have found a growing use in structural shear wall panels such as the Simpson Strong-Wall Shear wall shown in Simpson Strongtie® connector catalog by Simpson Strong-Tie Company, Inc. catalog C-2002 pages 27, 30, and 31 and in Mueller U.S. Pat. No. 5,706,626, granted Jan. 13, 1998.
Mueller continued to use a standard nut, but did increase the thickness of the washer to withstand the greater loads. Mueller, however, teaches only the thick square washer and has virtually no curvature in his holdown connector which give the benefits of force flow control.
As evidence of the successful use of the compound curve force flow feature of the present invention, a destruction pull test of the connector of the present invention caused the holdown to fail in the strap portion at a point above the compound curve; not in the curved portion adjacent the seat.
The work done by others in the field to improve the ability of concentric holdowns to resist huge upward forces due to earthquakes and high winds such as hurricanes and tornadoes generally resulted in raising the held column member needlessly high above the foundation member. Applicant has labeled this increase in the height of holdowns, especially concentric holdowns as set forth above, as the “tower effect”. The raising of the held column member or “tower effect”, required the designers who built the U-shaped members and other lifting blocks to increase the length and thickness of the side straps to hold the columns in an upright position and to prevent them from bending over laterally.
This “tower effect” problem has been greatly alleviated by the use in the present invention of a combination member in which a single member, due to its internal threads, holds the combination member to the anchor member and serves as a nut while its large area serves as a clamping surface.
By eliminating sharp bends in the holdown connector member of the present invention and the use of compound bending through cold working of the metal, and directing flow of forces by means of the combination member, the problem of premature failure of the holdown member at the joinder of the upright straps to the flat seat member has been eliminated.